1. Field of the Invention
The present invention relates to automatic rangefinder devices and, more particularly, to rangefinders of the multi-zone, active, triangulation type wherein a beam of light is projected on a scene to illuminate a spot on an object, and an array of photosensor elements sense the location of the spot in an image of the scene. The output signals of the photosensor elements are multiplexed for determining the distance to the object in the scene.
2. Description of the Related Art
In one type of automatic rangefinder, a beam of light is projected upon an object in a scene to form an illuminated spot on the object. An image of the scene, including the illuminated spot, is formed on a linear image sensor. The location of the illuminated spot along the image sensor is determined by the distance to the object in the scene. Such a rangefinding device finds application for example in an automatic focusing camera.
The general arrangement of elements and mode of operation of such a rangefinding device will be described with reference to FIG. 1. Means for projecting a beam of light is shown by way of example as an infra-red emitting diode (IRED) 10 and a lens 12. The beam is projected along a path 14 to illuminate a spot on an object O.sub.1 in a scene. The scene is imaged by a second lens 16 onto an image sensor comprising a linear array of photosensors 18. The signals produced by the photosensors are analyzed by control electronics 20 to determine the position of the illuminated spot in the scene and produce a signal representing the distance to the object.
As shown by example in FIG. 1, the apparent position of the illuminated spot in the scene is a function of distance along light path 14 to the object. For an object O.sub.1 located at a distance D.sub.1 from the rangefinding device, the image of the illuminated spot will fall on the image sensor at location S.sub.1. For an object O.sub.2 located at a further distance D.sub.2, the image of the spot will fall on the image sensor at location S.sub.2. By examining the output of the image sensor, the control electronics determines (for example, by comparing the outputs of the elements to determine that output which is a maximum) the location of the illuminated spot in the scene and thereby the distance to the object.
In U.S. Pat. No. 4,575,237, issued Mar. 11, 1986, a rangefinding device for measuring the distance to an object is disclosed. In particular, the '237 device is arranged to continuously measure the distance to the object by processing the signals obtained through a plurality of detectors. The output signal of each detector is compared to all other detector outputs and the control circuit computes a peak value. A disadvantage of such a rangefinder is the extra expense of the comparison and computational circuitry required to implement such a rangefinder. This would be undesirable in a low cost rangefinder application.
In U.S. Pat. No. 4,221,474, issued Sep. 9, 1980, a focussing system which time-multiplexes the signals from two photodiodes is disclosed. The '474 focussing system employs optics which transversely shifts in dependence upon exposure-objective subject-distance settings, onto one, the other or both of the two photodiodes for changing the field of view of each photodiode while determining focus adjustment. The signals of the photodiodes are routed to a peak detection signal processing stage employing amplitude comparison of the photodiode signals with a reference voltage. Use of the amplitude comparison makes the device susceptible to undesirable performance (i.e., variations in zone cross-over point locations) caused by variations in object reflectivity. Another disadvantage of this system is that it requires movement of its optical components, making it complex and requiring numerous components. Such a system is undesirable for implementation in a simple, low cost rangefinder device.
In U.S. Pat. No. 4,534,636, issued Aug. 13, 1985, an automatic focusing device for a camera is disclosed. The focusing device includes a rangefinder section having an array of detector elements and a logic circuit for detecting the closest object in a photographing field. The rangefinder section further includes amplitude comparison circuitry for each detector in which a reference voltage is compared with each detector signal. A disadvantage of such a device is that duplicate comparison circuitry is employed, thus adding to the expense of manufacturing the same. In addition, as a result of reference voltage comparisons with detector signals, the '636 device performance can be undesirably affected by variations in the reflectivity of the main object. The expense of duplicate circuitry and sensitivity to object reflectivity are undesirable in a low cost, reflectivity insensitive rangefinder.
In U.S. Pat. No. 4,527,891, issued Jul. 9, 1985, a rangefinder device having serial readout of a linear image sensor via a charge-coupled device shift register and a peak detector is disclosed. A disadvantage of the '891 rangefinder is the high cost of the charge-coupled device and associated electronics, resulting in a high total cost to manufacture and use the same. This is undesirable in a low cost rangefinder device.
In U.S. Pat. No. 4,582,424, issued Apr. 15, 1986, a distance measuring apparatus is disclosed. The '424 apparatus requires a plurality of light-emitting units and a plurality of light-receiving units to effectuate a ranging operation. A disadvantage of the '424 apparatus is the requirement of a plurality of light-emitting units to view different areas of the scene. This adds expense to the manufacture and use of such a rangefinder. The '424 apparatus further suffers in that it utilizes amplitude comparisons of the detector output signals to a threshold reference voltage. As a result, the apparatus is susceptible to undesirable operation due to variations in object reflectivity. These disadvantages are undesirable in a low cost, high performance (i.e., reflectivity insensitive) rangefinder.
It would thus be desirable to provide a multiple zone rangefinder which is low cost and reflectivity insensitive.